There is known from patent publication FI-95618 a drilling apparatus in which an outer circle of the hole drilling ring bit acts to drill an outer circle, so that a casing tube can be pulled into the hole in connection with the drilling. However, when the drilling apparatus is pulled out from the hole so that the casing tube remains in the hole, the ring bit also has to remain in the hole bottom.
Also from patent publication FI-85302 there is known a drilling apparatus for drilling large holes in which the center locating bit drills a hole center portion, and separately on the outer circle of the drilling apparatus there are placed bits which drill the outer circle of the hole. The outer circle drilling bits have rotating and hammering devices of their own. The drilling apparatus is used for drilling of horizontal holes, so that the drilling apparatus is meant ultimately to penetrate through the ground into free space.
It is well known where drilling is carried out by a rotating bit which is mounted eccentric in relation to a center axis of the casing tube, that it is possible to drill a larger hole than the diameter of the casing tube. When drilling is stopped, the eccentricity of the bit in relation to the center axis of the casing tube is changed, so that the bit can be pulled out from the hole and the casing tube is left in the hole.
The disadvantage for the drilling apparatuses in which the ring bit must be left in the hole is that there is then lost in every hole a relatively expensive bit. The disadvantage when an eccentric bit is used which must be lifted up from the hole is that the bit will wear very quickly because the drilling surface in the bit is substantially smaller than the drilling surface of the hole, so that the worn bits have to be changed often. In certain drilling apparatuses where the drilling bits can be pulled against each other thus allowing the bit assembly to be lifted up in spite of the casing tube, the disadvantage is that the mechanism by which the bits are pulled against each other has to be complicated, and it is difficult to change the bits and they can be very easily damaged.